Scorpene v/s. Agosta: Silent killers of the deep news
26 February 2008

The development of the Agosta
Going back in history, the first Agosta class submarine was launched as far back as 1974 as a precursor to the Rubis nuclear attack submarine. She had a crew of 58 personnel and had an armament of Exocet missiles and 533 mm torpedoes. The earliest users of this submarine were the French and Spanish navies. The Pakistan Navy also acquired two of them when the original customers, the South Africans, could not receive them due to the arms embargo caused by the nation's apartheid policy.

By late 1999 a total of 13 Agosta-class submarines had been ordered by France, Spain and Pakistan. While the original design incorporated a lot of modern technologies, more importantly, its inherent adaptability made it an excellent candidate for an upgrade.

The Agosta 90B is a direct derivative of the above but features significant improvements in acoustic discretion and detection, as well as considerable more automation compared to its predecessors. This has resulted in a reduction of the stipulated crew from 58 to 36. Other improvements include a new battery for increased range and a deeper diving capability of 350m resulting from the use of new materials such as HLES 80 steel. Another important improvement is compatibility with the MESMA AIP system, which has already been fitted onto the Hamza and will be added to the Khalid and Saad in the near future.

The Agosta 90B is equipped with a fully integrated SUBTICS combat system and can carry a mix of 16 wire-guided torpedoes and sea-skimming Exocet missiles. The submarine is fitted with a Thales Underwater Systems TSM 223 sonar suite, which includes bow-mounted sonar and towed sonar arrays, SAGEM periscopes and navigation system and Thales I-band navigation radar.

The propulsion system consists of two SEMT-Pielstick 16 PA4 V 185 VG diesels providing 3,600hp and a 2200kW electric motor driving a single propeller. As an enhancement, it's now fitted with the MESMA AIP system that allows the submarine to remain submerged three times longer. However, this results in an increase in the length and weight of the submarine, and also marginally reduces its maximum speed underwater.

The development of the Scorpene
The Scorpene is the latest submarine from the DCNS shop and is built in collaboration with Navantia of Spain. Ironically, the first one was built in 2003 at the same location where the first Pakistani Agosta 90B was constructed in 1999 the DCN shipyard at Cherbourg in France. It was named O'Higgins and acquired by Chile. The second, Carrera, built at the Cartagena shipyard of Navantia in Spain was launched in November 2004 and commissioned in July 2006.

The Malaysian Navy had also ordered two Scorpenes as far back as 2002, which are expected to be commissioned in 2009. The first one is being built in France and the second one in Spain. India placed an order for six of these submarines in October 2005, with the condition that they are built at the Mazegaon Shipyard in India under a Transfer of Technology programme. In addition, as armament for these vessels, the Indian Navy has also ordered 36 MBDA SM-39 Exocet anti-ship missiles. Construction on the first vessel started in December 2006,with delivery due in December 2012. The remaining five Scorpenes will be delivered over the next five years, at the rate of one a year.

The Scorpene is fitted with the SUBTICS combat management system, with up to six multifunction common consoles and a centrally situated tactical table. It is composed of a command and tactical data handling system, a weapon control system and an integrated suite of acoustic sensors with an interface to a set of air surface detection sensors and the integrated navigation system. The vessel has excellent detection capability due to its extensive sonar suite which includes a long-range passive cylindrical array, an intercept sonar, active sonar, distributed array, flank array, a high-resolution sonar for mine and obstacle avoidance and a towed array.

Where the Scorpene outscores the Agosta
The Scorpene has an increased diving depth of 370m as compared to the Agosta 90B's 320m, and has no limitations on operations at maximum depth due to the increased usage of high-yield stress-specific steel in its construction. Submerged, the Scorpene is quieter than most submarines due to the utilization of advanced hydrodynamics with an albacore bow shape, with fewer appendages and an optimised propeller. Placing systems on elastic mountings further minimizes noise.

Where the Scorpene particularly scores over the Agosta 90B is in its stealth capabilities. Unlike the Agosta, which is a modification of a three-decade old design, the Scorpene incorporates the latest in hydrodynamics, damping and stealth technologies to reduce its noise signature, thereby greatly increasing its detection capability and offensive power.

The submarine is manned by a standard crew of 31 (less than the Agosta 90B), with additional space for 6 special operations personnel. The entire operational and living area is air-conditioned, and the latest safety systems to allow for the operation of a Deep Submergence Rescue Vehicle (DSRV) are present. The propulsion system consists of two diesel generation sets providing 1,250kW of power coupled with a 2,900kW electric engine.

The Scorpene can also carry more weapons than the Agosta 90B a mix of 18 torpedoes and missiles or 30 mines. The six bow-located 21-inch torpedo tubes also have salvo launch capability, and DCNS has expressed its willingness to integrate the Brahmos cruise missile in its armoury as well, in addition to the Exocets already ordered.

While the original Agosta had not been designed keeping an AIP system in mind but had it included only in the 90B variant, the Scorpene has been designed to seamlessly integrate an AIP system. This results in increased submerged range steaming under AIP for the Scorpene as compared to the Agosta. The question now arises as to what exactly is an AIP system?

What is air-independent propulsion?
AIP stands for Air-independent Propulsion, and refers to technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen, and usually excludes nuclear power. The USP of a conventional diesel-electric submarine is its ability to remain undetected for long periods of time. This is primarily because of the quietness of its electric engine and ability to remain underwater for extended periods. Even though such submarines are powered by electric motors, driven by batteries, these batteries need to be regularly charged using diesel engines.

This ability of remaining undetectable is severely compromised if it has to repeatedly come to periscope or snorkel depth to recharge its batteries that drive the electric motor, using its diesel engine that requires air for operation. Not only can the snorkel be more easily detected, the noisier diesels may also make the submarine's presence known to the enemy. To overcome this problem, the submarine can be fitted with an AIP system that can temporarily charge the vessel's batteries or drive the electric motor.

The following types of AIP systems have been developed till date:

1. Closed cycle engines uses a conventional diesel engine with liquid oxygen as an oxidant for underwater operation. Not used due to safety concerns.
2. Stirling cycle engines closed-cycle regenerative hot-air engine using diesel and liquid oxygen. Currently used by the Swedish Navy. Heat from an outside source is transferred to an enclosed quantity of working fluid, generally an inert gas, and drives it through a repeating sequence of thermodynamic changes. By expanding the gas against a piston and then drawing it into a separate cooling chamber for subsequent compression, the heat from external combustion can be converted to mechanical work and then, in turn, to electricity.
3. Fuel cells developed by Siemens for use by the German Navy. Uses oxygen and hydrogen as reactants to produce electricity and water. Considered to be the quietest due to the lack of moving parts and exhaust gases, but safety concerns remain due to the dangers of storing highly inflammable volatile hydrogen in pressurised form.
4. Closed cycle steam turbines the French MESMA system is the only operational example. Burning ethanol with compressed oxygen at 60 atmospheres produces heat in the primary circuit. Although power output is highest of the four processes, it is also the least efficient requiring the maximum amount of oxygen, and is also noisier than fuel cells owing to the presence of moving parts and exhaust gases.

It is to be noted that AIP systems can provide only a fraction of the conventional propulsion system and can be used only for low-speed maneuvers. However, it does increase a submarine's capability to stay underwater at low speeds by several factors and hence form an indispensable part of the modern diesel-electric submarine.

The Indian Navy is yet to decide between Siemens PEM (polymer electrolyte membrane) fuel cells and the MESMA system for its six Scorpenes, though the latter seems to be at an advantage considering it comes from the same manufacturer as the submarines themselves. The German system may be fitted onto the existing Shishumar class boats of the Indian Navy.

Variants of the Scorpene
The Scorpene class submarines come in three subtypes: Basic, Basic-AIP (with MESMA air-independent propulsion) and Compact. The Scorpenes used by the Chilean Navy are of the first type. India is most probably ordering the second type.

Variants Basic Basic-AIP Compact
Overall length 66 metres 76 metres 60 metres
Submerged displacement 1700 tons 2010 tons 1450 tons
Maximum submerged speed 20 knots 19 knots 14 knots
Diving depth 370 metres 370 metres 250 metres
Crew complement 31 31 22
Endurance 45 days 45 days 40 days

In conclusion, it can be said that though the Indian Navy is receiving a better product vis--vis its traditional adversary, it must make sure that the product is delivered as per schedule, and meets all original specifications, especially the ''transfer of technology'' part. At the same time, it must ensure, in conjunction with other agencies like the Atomic Energy Commission (AEC) and the Defence Research and Development Organisation (DRDO), to have a working prototype of the Advanced Technology Vessel (ATV) at the earliest. It must always be kept in mind that ''a bird in hand is worth two in the bush'', and the Pakistan Navy already has three of them in hand.

Here is a comparison table of the MESMA AIP variants of the Agosta 90B and the Scorpene. Some of these figures may not be entirely accurate, but that is only expected of top-secret military projects.

Parameters Agosta 90B Scorpene
Overall length 76 metres 76 metres
Overall width 6.8 metres 6.2 metres
Overall height 5.4 metres 5.8 metres
Surfaced 1810 tons 1850 tons
Submerged 1980 tons 2010 tons
Surfaced 13,700 km 10,500 km
Submerged 2,250 km 3,500 km
Maximum Speed    
Surfaced 12 knots 12 knots
Submerged 19 knots 19 knots
Diving endurance 60 days 45 days
Diving depth 320 metres 370 metres
Complement 36 (7 officers) 31 (6 officers)
Weapons 16 missiles and torpedoes 18 missiles and torpedoes

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Scorpene v/s. Agosta: Silent killers of the deep